Method for manufacturing varistor

Abstract

The raw material for a ZnO—Pr-based varistor is mixed with cobalt and potassium to prepare a ceramic raw material powder. Potassium, which is an alkali metal, is added in the form of KClO4, KHC4H4O6, PtCl6, or K3[Co(NO2)6]. The ceramic raw material powder is pulverized by a wet process while 0.1 to 5.0 wt % of polycarboxylate dispersant is added to the powder. Such a method can produce a reliable, high-quality varistor having excellent varistor characteristics, such as varistor voltage V1mA and insulation resistance IR, and less variation in these characteristics.

Description

TECHNICAL FIELD [0001]The present invention relates to varistor manufacturing methods and varistors. In particular, the present invention relates to a varistor manufacturing method including a step of mixing zinc oxide (ZnO), as the major component, and a compound containing potassium (K), as a minor component, and pulverizing the mixture by a wet process, and also relates to a varistor manufactured by this method.BACKGROUND ART [0002]The major components of well-known varistor compositions include SiC, SrTiO3, and ZnO such as ZnO—Bi and ZnO—Pr. Among them, ZnO—Bi-based and ZnO—Pr-based varistors are becoming widespread as surge protectors. ZnO—Bi-based and ZnO—Pr-based varistors, having more excellent voltage nonlinearity and surge current capability than SiC-based and SrTiO3-based varistors, deliver high ability to protect electronic devices from surge currents.[0003]Between these two types of ZnO-based varistors, ZnO—Pr-based varistors exhibit excellent voltage nonlinearity but p...

AI Technical Summary

Benefits of technology

[0014]According to the present invention, potassium, which is added in the form of a poorly soluble potassium compound, does not dissolve in a first slurry containing water as a solvent in the mixing and pulverizing step, causing no compositional variation. In addition, such a compound, which does not dissolve in the first slurry, does not vary the pH of the first slurry, preventing an increase in the slurry viscosity and decreases in the miscibility, millability, and dispersibility. Furthermore, such a compound can prevent the aggregation and nonuniform precipitation of potassium through recrystallization during the drying. The present invention, therefore, can provide a method for manufacturing a reliable varistor that can operate at low voltage.

[0015]In this method, a compound containing praseodymium and a compound containing cobalt are preferably used as additional minor components. These compounds serve to reduce the leakage current of the varistor.

[0016]To enhance the above effects, the solubility of potassium contained in the poorly soluble potassium compound used is preferably 3 g or less in 100 g of aqueous solution at 25° C. Preferred examples of the poorly soluble potassium compound used include at least one material selected from the group consisting of potassium perchlorate, potassium bitartrate, potassium hexachloroplatinate, and potassium hexanitrocobaltate.

[0018]Furthermore, an intensive study by the present inventor has found that a dispersant added in the mixing and pulverizing step can homogeneously disperse the components in combination with the poorly soluble potassium compound to prevent defective grain boundaries. The study has also found that the dispersant added is preferably a polycarboxylate dispersant in view of the environment and the durability of the firing furnace used.

[0023]During the manufacturing process, potassium does not dissolve or is not removed, providing excellent dispersibility. The present invention, therefore, can provide a reliable varistor that can operate at low voltage and that has excellent varistor characteristics.

Problems solved by technology

However, the voltage nonlinearity decreases by defective grain boundaries, which may increase by, for example, nonuniform precipitation of added elements and the yield of compounds from the added elements.

Such an impairment in the ESD characteristics results in decreases and variations in varistor voltage Vow and insulation resistance IR.

The compositional variation makes it difficult to provide a high-quality varistor having excellent and stable varistor characteristics.

Then, the slurry fails to maintain its dispersion system.

As a result, the particles aggregate to increase the viscosity of the slurry, leading to decreases in the miscibility, millability, and dispersibility of the slurry.

Furthermore, during drying after the dehydration, potassium readily aggregates and nonuniformly precipitates through recrystallization to decrease the dispersibility.

This may result in further impairment in the varistor characteristics.

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